Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay is widely used as a filler and a pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin clay pigments in paper coatings serves, for example, to improve the opacity of the coated paper due to the light scattering ability of the kaolin clay pigment. Light scattering ability (or opacity) of the pigment can be estimated by the scattering coefficient as described in TAPPI, 1978, Vol. 61, No. 6, pages 78-80.
The light scattering ability, or opacifying characteristics, of kaolin clay may be enhanced by aggregating or structuring the minerals of the kaolin clay by chemical or thermal treatment as described above.
Despite their high opacifying characteristics, the usefulness of chemically-structured kaolin clay is limited by its poor rheological properties. In this application, the term "rheological properties" refers to the viscosity or flow characteristics of the structured clay materials, including structured kaolin clay.
The high viscosity of structured kaolin days disclosed in the prior art reduces their pumpability and poses problems in transferring the structured kaolin clay. Further, the high viscosity limits the use of structured kaolin clay in paper coating applications, especially in the high speed paper coating processes.
Therefore, a need exists in the kaolin industry for an opacifying kaolin clay pigment with improved rheological properties.